Implications of the variation in microstructure caused by changes in helium generation rate and other irradiation parameters

The effect of changes in helium generation rate on void and bubble nucleation, growth and distribution observed mainly in high-purity aluminium are summarized. It is shown that at a given irradiation temperature, the cavity microstructure may be composed of voids only, voids and bubbles or bubbles only, depending on helium generation rate. The accumulation of helium (generated during irradiation) on grain boundaries is considered; the helium generation rate is found to control the flux of helium to grain boundaries and therefore may also determine the rupture lifetime. The influence of materials variables on the flux to the boundaries is also considered. Changes in recoil energy and/or damage rate may have strong influence on precipitation and precipitate stability-and hence on helium flux to grain boundaries and mechanical properties. The implications of the effects of changes in irradiation parameters and materials variables are discussed. (author)